Chair base



FIG. 1 is a top plan view of a chair base showing my new design;

FIG. 2 is a bottom plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof; and,

FIG. 5 is a side elevational view thereof, with the other sideelevational view being a mirror image.

The ornamental design for a chair base, as shown and described.